Circuit boards embodying one or more components are usually tested at least once during manufacture to determine whether the board is operating properly. Often, the board is subjected to an in-circuit test during which the operation of each component on the board, such as an integrated circuit or the like, is tested by monitoring the response of the component to various test signals input thereto. During in-circuit testing, it is desirable to isolate the component under test from changes in the signals supplied by integrated circuits on the board not under test. Otherwise, such changes in signals from the integrated circuits not under test may adversely affect the test results.
A common way in which the component under test is isolated from such signal changes is to back drive the integrated circuits which supply the signals to the component. Back driving of each integrated circuit is accomplished by either sourcing current to, or sinking current from the output of the integrated circuit, depending on whether the output signal is at a logic high or logic low state, respectively. In this way, the output signal of the integrated circuit can be held at a particular logic level, regardless of the input to the integrated circuit. Thus, the output signal supplied from the integrated circuit to the component under test may be held at a known level.
Often it is desirable to test each component while the input signals supplied thereto from the integrated circuits not under test alternate between a high and low state at a rate near the operating frequency of the component under test. If the output signals of the integrated circuits are to alternate at a given rate, notwithstanding the state of the input signal supplied thereto, then current must be alternately sourced to, and sunk from the output of the integrated circuits at such a rate. In some instances, the operating frequency of the components on the board is 100 MHz or greater, requiring current to be alternately sourced to, and sunk from the integrated circuits not under test at the same rate.
Conventional driver circuits exist for alternately sourcing current to, and sinking current from a load, such as an integrated circuit. Most conventional driver circuits, however, cannot alternately source and sink current at a rate much above 20 MHz because of the manner in which they are constructed. Conventional driver circuits usually have an output stage comprised of a pair of bipolar transistors which have their collector-to-emitter portion coupled between a separate one of a pair of positive and negative voltage sources, respectively, and the load to be back driven. The conductivity of each of the first pair of transistors in the output stage is controlled by an input signal supplied by a separate one of a second pair of bipolar transistors. The transistors of the second pair are each supplied at their base with a separate one of a pair of square wave signals 180.degree. out of phase with each other so that the transistors are alternately rendered conductive. As a result, the transistors of the first pair are alternately rendered conductive, causing current to be alternately sourced to, and sunk from the load.
It is desirable to operate the above-described prior art driver circuit such that each of the first pair of transistors in the output stage is driven into and out of saturation. In this way, the output current of each of the first pair of transistors will not undesirably vary linearly with the input current supplied by a separate one of the second pair of transistors. However, the disadvantage of operating the first pair of bipolar transistors in their saturated state is that the switching speed of the transistors will be limited, limiting the rate at which current can be alternately sourced to, and sunk from the load.
One way to increase the operating speed of a conventional driver circuit would be to substitute a high-speed metal semiconductor field effect transistor (MESFET) in place of each of the first pair of bipolar transistors. The difficulty with this approach is that the transistors of the second pair in the conventional driver circuit, which would control the conductivity of a separate one of the MESFETs, are typically operated so the collector current in each never approaches zero. However, to assure that the MESFETs would turn on strongly, the transistors of the second pair in the conventional driver circuit would have to operate in their cutoff region, which would severely degrade the performance of such a driver circuit. For this reason, field effect transistors have not been widely used in conventional driver circuits.
Thus, there is a need for a driver circuit capable of alternately sourcing and sinking current at a very high rate.